Various liquid micro-dispensers, or pipets, known in the prior art suffer disadvantages which the present invention overcomes.
For example, U.S. Pat. No. 4,418,580 to Stachell et al discloses a dispenser which includes a hollow tip adapted for the attachable connection to the dispensing end of the dispenser, a piston member slidably disposed within the tip, a plunger for selective engagement with the piston member, and an ejector for rapid removal of the tip and the piston without the user contacting these components.
However, with the Satchell et al dispenser, the ejecting conical member 126 is fixed relative to the tip-and-piston assembly 32 during the tip ejecting procedure. Thus, the piston 36 is restricted to a fixed location in the dispensing tip in order to facilitate its detachment during the ejection procedure.
Further, the eject mechanism requires the use of an additional device (a side button) to lock the plunger shaft in an extended, full volume position.
Moreover, no mechanism is provided for permitting the user to adjust the dispensing volume since volume settings are fixed when the unit is assembled, and no mechanism is provided for volume calibration of the instrument if it were possible to use tips having different volumes with the patented pipet. Since each instrument is designed and pre-calibrated for a given volume to be dispensed from a given tip, it does not appear that the patentees contemplated using more than one size tip to fit any given instrument.
Also known in the prior art in U.S. Pat. No. 4,362,064 to d'Autry which discloses a high-precision positive-displacement pipet having a nozzle with calibrated capillary channel means capable of being removably engaged with the free lower end of a pipet body. The dispenser of the d'Autry patent employs an entirely different mechanism for ejection of the nozzle which consists of a gripping device 32 having resilient arms 34 which engage a shouldered gripping element 16 at the upper end of a piston 18. By way of this structural relationship, there is provided an external engagement of the plunger shaft with the piston located within the nozzle.
However, the d'Autry patent has the disadvantage that no provision is made for automatic calibration of the dispensing nozzle (tip) and piston. Minor variations in length of the tip and the piston, generally exhibited with molded parts, and especially molded parts which are made of plastic and which may be subjected to sterilization in a steam autoclave and therefore are subject to shrinkage, are not accommodated by the d'Autry dispenser because the piston 18 always engages gripping device 32 against the shoulder of gripping element 16. The total length of the plunger (controller rod 24) and the piston are not adjustable during installation of the piston, and thus tip-to-tip errors can occur due to molding tolerances.
Still other conventional pipet-type dispensing devices are known. For example, U.S. Pat. No. 4,023,716 to Shapiro discloses the disengagement of a dispensing tip from its snap-fit connection in response to axial force imparted by a user to a plunger. U.S. Pat. No. 3,918,306 to Maltby discloses the disengagement only of a tip portion of a pipet-type dispensing mechanism. U.S. Pat. No. 3,760,639 to Sokol et al shows a disposable tip and piston. All of these additional related patents suffer from the same deficiencies and disadvantages discussed above in relation to the d'Autry and Satchell et al patents.